1. Field of the Invention
The present invention relates to semiconductor devices for image sensing and capturing, and more specifically to Very Large Scale Integration (VLSI) semiconductor technology apparatus comprising Active Pixel Sensing (APS) arrays.
2. Description of the Related Art
A photo-transistor, unlike a photo-diode, is a high output impedance (light-controlled) current source. Also, the output impedance of a photo-transistor can be rendered independent of the size of its photosensing junction, while the output impedance of a photo-diode cannot. Because of these two properties, photo-transistors are more effective sensors than photo-diodes in certain applications. Such applications include those in which the voltage output of the loaded sensor is limited by the impedance of the sensor rather than the dynamic range of the load, and those in which response linearity is important, but where response linearity is determined by the impedance of the sensor rather than the impedance of the load.
Bipolar photo-transistors have been known and used extensively in discrete and integrated circuit applications. However, it is generally difficult to isolate electrically a bipolar photo-transistor from adjacent structures in an integrated circuit. It is also generally difficult to keep the base recombination current of these transistors from reducing conversion efficiency. The photo-transistors of the present invention are inherently free of these serious limitations.
Further objects and advantages of this invention will become apparent from the detailed description which follows.